1. Field of the Invention
This invention relates to improving the anticorrosive properties of an autodeposition coating by a post-bath rinse which introduces certain metal salts into the resin coating.
2. Statement of the Related Art
Inorganic pigments (i.e. insoluble colored substances) are capable of improving the anticorrosive properties of coatings which have been autodeposited on metal substrates. This is known from various publications. U.S. Pat. No. 4,030,945 discloses a process wherein metal surfaces, after they are autodeposition coated with organic resins, are rinsed with diluted aqueous solutions containing hexavalent chromium or combinations of hexavalent chromium with formaldehyde-reduced chromium compounds. In accordance with the disclosed process chromium compounds such as chromium trioxide (chromic acid), and/or water or acid soluble chromates or dichromates can be employed, especially potassium or sodium dichromate, or sodium, potassium, or lithium chromate.
Improving the corrosion resistance of metal surfaces autodeposition coated with resins has also been proposed in U.S. Pat. No. 4,186,226, which discloses using alkali metal chromates or dichromates in a post-bath rinse.
U.S. Pat. No. 3,647,567 discloses what appear to be autodeposited resin coatings which are subjected to a post-bath rinse using chromium trioxide, phosphoric acid, or water soluble or acid soluble chromates and dichromates. The specifically disclosed chromates or dichromates are: potassium, sodium, ammonium, calcium, cesium, lithium, magnesium, zinc, etc. (sic) chromates and sodium, ammonium, lithium, etc. (sic) dichromates, zinc chromate being preferred. In the only specific disclosure of such salts, (Example 3), a zinc "chromate" containing solution was prepared by adding an excess of zinc carbonate to a 10% aqueous chromium oxide solution. In the coating bath, various resin latices are disclosed, but the activating systems are oxidizing acid systems, specifically nitric acid or sulfuric acid when the substrate contains iron, zinc, or tin. A mixture of fluoroboric acid, hydrofluoric acid, chromic anhydride and potassium ferricyanide can also be employed as the activating system.
Theoretically, it is also possible to add inorganic pigments improving the autodeposition coating anticorrosive property directly into the coating bath and deposit same along with the organic resin layer on the metal surface. Known anticorrosive pigments include compounds of barium, strontium, zinc and lead, the chromates of said metals being preferably used. Such chromates without exception are only sparingly soluble in water. As is known from numerous printed publications, the autodeposition method is such that the acidic latex superficially mordants the metal surface to be coated, thereby dissolving metal ions of the metal surface into the solution. Such positive charge carriers cause the stabilized resin dispersion to coagulate in the proximity of the metal surface, whereby a homogeneous coating with the organic resin is effected without electricity. Due to the low pH of this coating process (between 1.5 and 4.0) such anticorrosive pigments are more or less rapidly converted into a soluble form, and theoretically should then be deposited simultaneously with the organic resin particles. However, the metal cations present in the acidic aqueous solutions contribute to an increased coagulation of the resin dispersion which may even result in a breakdown of the latex due to its complete coagulation.